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compression ratio |
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chainsaw39 
AMC Apprentice 
Joined: Jun/02/2009 Location: Butte Montana Status: Offline Points: 41 |
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 Topic: compression ratioPosted: Jun/16/2009 at 11:56am |
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Hi probably a stupid question but here goes. To determine comp ratio can I simply take cylinder pressure in psi and divide by atmospheric pressure in my location? I am trying to determine if my amx is running stock compression. All 8 cyl. read 135 to 137psi atmospheric pressure here is 12. Any help is greatly appreciated.
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1969 amx 390 auto go-pak
1970 rebel machine 4spd 1973 pierre cardin javelin 401 |
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chainsaw39
AMC Apprentice
Joined: Jun/02/2009 Location: Butte Montana Status: Offline Points: 41 |
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Posted: Jun/16/2009 at 1:06pm |
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I guess I would also have to subtract 1 atmosphere from the gauge reading.
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1969 amx 390 auto go-pak
1970 rebel machine 4spd 1973 pierre cardin javelin 401 |
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whizkidder
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Joined: Mar/03/2008 Location: North Georgia Status: Offline Points: 2972 |
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Posted: Jun/16/2009 at 3:08pm |
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I don't believe you can rely on cranking pressure to compute CR. The intake valve would have to close exactly at BDC, and you would have to have perfect sealing of the rings.
I'm not smart enough to figure out whether your math would be correct.
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Ron Frost
marne1ancient @ gmail.com 910 nine two two 0563 "There is no limit to what a man can do, so long as he does not care a straw who gets credit for it. Charles Montague |
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billd
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Forum Administrator Joined: Jun/27/2007 Location: Iowa Status: Offline Points: 30894 |
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Posted: Jun/16/2009 at 3:37pm |
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You'd have to compensate for pumping losses, any leakage, etc. -
Did you do a compression test with the choke and throttle held fully open? The valve would have to be fully open, unrestrictive as soon as the piston started down, then close only at the point the piston stopped moving, then lose nothing as it compressed. If you have a stock engine, isn't it easier to look it up? If it's not stock, use one of the several web sites to plug in your numbers and calculate it? |
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Ken_Parkman
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Joined: Jun/04/2009 Location: Ontario Status: Offline Points: 1813 |
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Posted: Jun/16/2009 at 3:56pm |
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Cranking cylinder pressure is hugely influenced by the camshaft. Different cams have the valve closing with the piston further up the bore and losing a bunch of volume, so you can't calculate compression based on cranking pressure. This is where you need to know what pistons are in the engine.
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chainsaw39
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Joined: Jun/02/2009 Location: Butte Montana Status: Offline Points: 41 |
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Posted: Jun/16/2009 at 3:56pm |
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I do not know if the motor is stock or not and I do not have all the receipts for the rebuild. I did have the throttle wide open when doing the test. I was just curious if my numbers would be in the ballpark.
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1969 amx 390 auto go-pak
1970 rebel machine 4spd 1973 pierre cardin javelin 401 |
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17tamx
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Joined: Dec/23/2008 Location: Peoria, AZ Status: Offline Points: 1170 |
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Posted: Jun/21/2009 at 4:34am |
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If this motor has been rebuilt and is still pretty fresh than you have a problem with this engine combination. A stock AMC V8 with 8.5:1 compression ratio should be making at lest 155 psi. When you think of compression ratio it is just the difference of the volumn of the cylinder and head with the piston at bottom dead center compared to the volumn of the cylinder and head at top dead center. What ready makes an engine run good is cylinder pressure and good flow of gases in and out of the combustion chambers. If this motor was rebuilt with stock pistons and a long duration cam was installed then the intake valve will be closing way after the piston has started back up the hole and the effective compression ratio will be greatly less then the calculated compression ratio.
You really should use a leak down tester to determine the condition of the sealing ability of the rings, valves and headgaskets. Excessive leak down can not be fixed with tuning and can only be fix with mecanical repairs. What you do with a leak down tester is screw the adapter into the spark plug hole and then pressurize the hole with shop air when both valves are fully closed. The unit has two pressure gauges, one to show the cylinder pressure and the other to show your air pressure in. You are watching for leak down and listening for the air that is escaping. Air at the carb is an intake valve, air in the crankcase is rings and air in the exhaust is an exhaust valves. If the leak down is 2% you have a great engine but it still may have too low a compression ratio for the cam. If you have 25% leak down then the motor needs to be rebuilt if you want a high performance engine. If you have air leaking at the carb or exhaust and not the crank case then rebuild your heads. Most modern engines have very little leak down %. Many people add Cam, Intake, Carb and Headers hoping to pick up a bunch of power only to find that they do not have enough squeeze to make it all work. Hope this helps.
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Kirk P. Fletcher
70 AMX BBG w/Shadow 390 4sp 71 SC360 Wild Plum Ram Air 4sp 67 Rogue Convert 343 4sp 66 Rogue Hardtop 290 Auto 66 440 Convert 232 Auto |
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farna
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Moderator Lost Dealership Project Joined: Jul/08/2007 Location: South Carolina Status: Offline Points: 19675 |
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Posted: Jun/21/2009 at 12:36pm |
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The older TSMs don't give a cylinder pressure spec, but 74+ does. All 74-79 TSMs (yes, I checked them all!) state that the minimum cylinder pressure should be 140 psi for all V-8s. Stock 304s had 8.4:1, 360/401 had 8.25:1. Of course that's with the stock cam. Note that this is the MINIMUM cylinder pressure, 155 psi should be about right for a fresh low mile engine. Since you're at a high altitude and have 2.7 psi less atmospheric pressure, I'd say your engine would develop more than the minimum 140 psi at sea level since you're very close to that now. I don't know how to correct for altitude though, never really had reason to bother.
You can't rely on cylinder pressure for an accurate compression ratio. There are really two compression ratios, one is static (which is what you're referring to) and the other is dynamic. Static compression ratio (SCR) is the physical difference between the volume of the cylinder with the piston at the bottom and at the top with both valves closed. Dynamic (DCR) is the effective compression ratio with the engine running, and takes into account the cam used. Most engine actually run with a DCR in the 7-8 range, even though they have a SCR up around 8-11. This site is on Harleys, but the calculator works for any engine. It has a good explanation of DCR and how a cam affects it. http://www.rbracing-rsr.com/comprAdvHD.htm |
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Frank Swygert
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RB401
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Posted: Jun/21/2009 at 12:55pm |
I can't answer your question, but its worth noting that you have very uniform cylinder pressure which is a good indicator that the engine is in good condition. As the previous poster mentioned, your at high altitude which may account for the lower than expected peak pressure values. Given the pressures you are getting and considering the altitude it seems like you have a good engine with the stock compression ratio of about 8.5:1. One other indicator is : How does it run? If you have nice smooth idle, I'd say you have a stock cam and are good to go. If you have noticable lope at idle and the engine doesn't really start to pull until higher rpms, you probably have a long duration cam which you may want to swap out for something closer to the stock profile that would work better with 8.5 compression.
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'73 Javelin AMX, 401, T5
T5 Swap Page: http://mysite.verizon.net/amxjavelin/t5swap/t5swap.html |
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chainsaw39
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Joined: Jun/02/2009 Location: Butte Montana Status: Offline Points: 41 |
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Posted: Jun/21/2009 at 5:38pm |
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Thanks for all the posts they were very informative.
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1969 amx 390 auto go-pak
1970 rebel machine 4spd 1973 pierre cardin javelin 401 |
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chainsaw39 wrote: